a) Field of the Invention
This invention relates to an exhaust gas heating system for an in-cylinder injection internal combustion engine, in which fuel is injected directly into a combustion chamber, so that heating of exhaust gas from the engine is effected by controlling the injection of fuel. In particular, this invention is concerned with an exhaust gas heating system for an in-cylinder injection internal combustion engine, which is suited for use in heating an exhaust gas purification device (especially, a lean NOx catalyst).
b) Description of the Related Art
As a method for heating exhaust gas in an in-cylinder injection internal combustion engine of the type that fuel is injected directly into a combustion chamber, a technique is proposed, for example, in Japanese Patent Application Laid-Open (Kokai) No. HEI 4-183922 that with a view to promptly activating a catalyst, the catalyst is promptly heated to an activation temperature by re-actuating a fuel injection valve to perform injection of additional fuel while an intake valve is still closed, namely, in an expansion stroke or exhaust stroke of the engine and then re-energizing a spark plug in the combustion chamber for the additional fuel to achieve ignition of the additional fuel or by mixing additional fuel into exhaust gas and igniting the additional fuel by a spark plug arranged in an exhaust passage.
Further, a technique is also proposed in Japanese Patent Application Laid-Open (Kokai) No. HEI 8-100638 that the temperature of exhaust gas is raised to activate a catalyst by performing an additional fuel injection (second fuel injection) in an initial to intermediate stage of an expansion stroke of primary combustion after an injection of primary fuel (first fuel injection) and its ignition, igniting the fuel, which has been injected by the second fuel injection, by a flame propagated from the primary combustion and further by performing a further additional fuel injection (third fuel injection) at a timing that the fuel, which has been injected by the further additional fuel, can be ignited by a flame propagated from the combustion by the second fuel injection.
According to the second-mentioned technique, the injection start time of the second fuel injection (additional fuel injection) is set at an optimal time immediately before the primary combustion by the first combustion injection (primary fuel injection) ends, that is, within a range from 10.degree. to 80.degree. after top dead center (an optimal range of the injection timing for the second injection is from 30.degree. to 60.degree. after top dead center), so that the additional fuel by the second fuel injection is ignited by a flame propagated from the primary combustion to raise the temperature of exhaust gas for the activation of the catalyst.
In addition, the second-mentioned technique is also designed to keep the specific fuel consumption low by choosing the heating of exhaust gas based on the second fuel injection or the heating of exhaust gas based on retardation of the ignition timing of the primary combustion as needed depending on the value of a target temperature of exhaust gas set by an exhaust-gas-heating-method-selecting unit. Specifically, it is controlled to perform the second, i.e., additional fuel injection when the setting of the target temperature of exhaust gas is 300.degree. or higher and to perform the retardation of the ignition timing for the primary combustion when the setting of the target temperature of exhaust gas is lower than 300.degree..
According to the technique of Japanese Patent Application Laid-Open (Kokai) HEI 4-183922, however, the spark plug in the combustion chamber is re-energized or the additional spark plug is arranged in the exhaust passage, so that the additional fuel is caused to burn to heat the exhaust gas. Re-energization of the spark plug in the combustion chamber is however accompanied by drawbacks that an ignition control logic becomes complex and no sufficient energy can be assured for the second ignition. Any attempt to assure sufficient ignition energy involves a problem that use of a larger igniter becomes indispensable, leading to higher manufacturing cost. On the other hand, the use of the spark plug arranged in the exhaust passage for the ignition of the additional fuel is also accompanied by a problem that it leads to an increase in the number of parts and also to an increase in manufacturing cost.
The technique of Japanese Patent Application Laid-Open (Kokai) No. HEI 4-183922 is designed to ignite the additional fuel by using the spark plug arranged in the exhaust passage. In an in-cylinder injection internal combustion engine, however, the fuel injected by a primary fuel injection usually undergoes substantially complete combustion during an expansion stroke and an exhaust stroke so that co and HC, which may become combustible sources, remain only at low concentrations within the cylinder. Moreover, in the case of a fuel which is low in self-ignition property (for example, gasoline), substantial energy (for example, heat, pressure, temperature and/or the like) is needed to achieve combustion. Even when the fuel injection valve is simply re-activated in an expansion stroke or exhaust stroke to perform an injection of additional fuel and the additional fuel is then ignited by the spark plug as in the conventional technique, there is a potential problem that sufficient energy may not be produced and combustion of the additional fuel may not take place accordingly. The heating of exhaust gas therefore may not be assured, resulting in a potential problem that the catalyst may not be promptly activated.
According to the technique of Japanese Patent Application Laid-Open (Kokai) No. HEI 8-100638, the injection quantity and timing of the second fuel injection are determined from a basic fuel injection quantity of the first fuel injection and an engine speed. In other words, the injection quantity of the second fuel injection is determined based on the basic fuel injection quantity of the first fuel injection, and in regard to the injection timing of the second fuel injection, a map is stored beforehand by an experiment while using the basic injection quantity of the first fuel injection and the engine speed as parameters, and an optimal injection timing corresponding to operation conditions is determined from the map.
Since the flame lasting duration of the primary combustion by the primary fuel injection (the first fuel injection) varies due to changes in various parameters of the engine (for example, the engine temperature, the exhaust gas recirculation rate at the time of the primary combustion, the air/fuel ratio (A/F) at the time of primary combustion, the ignition timing at the time of the primary combustion, etc.), the additional fuel injected by the additional fuel injection (the second fuel injection) may not be surely ignited by the flame propagated from the primary combustion in some instances.
Further, according to the technique of Japanese Patent Application Laid-Open (Kokai) No. HEI 8-100638, either the heating of exhaust gas by the second fuel injection or the heating of exhaust gas by retardation of the ignition timing of the primary combustion is chosen as needed depending on the value of the target temperature of the exhaust gas set at the exhaust-gas-heating-method-selecting unit. Even when additional fuel is injected or the ignition timing of primary combustion is retarded, the resulting thermal energy may be used in a significant proportion for the work required to depress the piston (gas expansion work) in some instances. This makes it impossible to effectively use the resultant thermal energy for heating the exhaust gas, leading to a problem that the temperature of the exhaust gas cannot be raised efficiently.